The genesis of multicellular organization in D. discoideum involves a morphogenetic sequence whose temporal, spatial and quantitative precision approaches that encountered in Metazoan Systems. Our ultimate aim is to account for the regulation of morphogenesis in this organism by identifying the specific morphogens involved and by characterizing in molecular and topographic terms their mode(s) of action. Two endogenous metabolites, 3'5'-cAMP and NH3 have been shown to play major roles in triggering and modulating specific morphogenetic events. In addition NH3 has been found to act as a reversible inhibitor of intracellular cAMP accumulation. The two effects operate over the same range of (NH3 + NH4+) concentrations and display the same pH dependency. Our immediate aims are: (1) To further test and extend the hypothesis that NH3 acts as a morphogen by controlling cAMP production; this will involve the study of a series of mutants that display morphogenetic derangements ascribable to abnormal sensitivity to NH3; (2) To determine the stie at which NH3 acts as an inhibitor of cAMP production and to characterize the mechanism of action; (3) To test some predictions of a recently proposed specific model of morphogenetic regulation which is based on the assumptions that cAMP production is limited to specific sites within the multicellular aggregate by the distribution of NH3 (governed by rates of production, dissipation and pH gradients) and that these cAMP sites, by chemotactic attraction, govern cell movements within the aggregate.